The use of encapsulated, wireless sensors within a patient's body is known in the art. Typically, such sensors include a sensing circuit, for detecting a parameter of interest, and a transmitter, with a suitable antenna, for transmitting readings of the parameter to a receiver outside the body. In some applications, the sensor may also be capable of receiving signals and/or radiated power transmitted from outside the body. The parameter detected by the sensing circuit may be a physiological parameter, such as a local pressure, temperature or chemical characteristic (such as pH), or it may be a position parameter, indicative of the position coordinates of the sensor. Generally, all the elements of the sensor are encapsulated in a sealed casing (also referred to as a package or “can”) made of a suitable biocompatible material, such as a ceramic or plastic material.
An exemplary wireless position sensor of this sort is described in U.S. Pat. No. 6,239,724, to Doron et al., whose disclosure is incorporated herein by reference. This patent describes a telemetry system for providing spatial positioning information from within a patient's body. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.
The strength of the signals transmitted by a wireless sensor depends, inter alia, on the size of the antenna that is used to transmit the signals. The ability of the sensor to receive signals is likewise a function of the size of the antenna. Unfortunately, the area of the antenna is limited by the available volume within the sensor casing. For implantable devices, in particular, this volume is very small, and the transmission range of the device may be very short as a result. In response to this problem, U.S. Pat. No. 6,009,350, to Renken, whose disclosure is incorporated herein by reference, suggests using multiple antennas connected in parallel with each other. The antennas may be mounted within the can of the implantable device or externally.
As another example, U.S. Pat. No. 6,053,873, to Govari et al., whose disclosure is incorporated herein by reference, describes a stent having a sensor for measuring characteristics of a fluid flow passing through the stent and a transmitter for transmitting flow parameters to a receiver outside the body. The stent comprises a coil, which also serves as an antenna, for receiving energy from an electromagnetic field irradiating the body so as to power the transmitter. In one embodiment, the transmitter is based on a tunnel diode oscillator circuit, suitably biased so as to operate in a negative resistance regime.